The present invention relates to a wavelength-converted light generating device and an irradiation method therefor, capable of being used in medical applications, precision machining and various types of measurements.
As examples of lasers which have been conventionally used in the medical field, there are discharge-pumped carbon dioxide gas (CO2 lasers), flash lamp-pumped neodymium ion-doped yttrium-aluminum-garnet (Nd:YAG) lasers and flash lamp-pumped erbium ion-doped yttrium-aluminum-garnet (Er:YAG) lasers.
Since CO2 lasers oscillate at 9-10 xcexcm, and have an absorption coefficient which is well-suited to organic soft tissue, they are often used as laser scalpels. However, they have the property of creating carbonized layers when cutting soft tissue, as a result of which the treated parts can be slow to heal. Additionally, since the lasing medium is CO2 gas, the laser energy capable of being extracted per unit volume is small, so that the laser head must be made rather large, thereby forcing the CO2 laser treatment devices themselves to be large.
On the other hand, Nd:YAG lasers have an oscillation wavelength of 1 xcexcm, so that their absorption coefficient for organic tissue is extremely small, and they have come to be used mainly for coagulation of soft tissue. When used for cutting, a special blade-shaped optical component known as a contact tip is used to convert the laser beam into heat along the boundary between the soft tissue and the contact tip as the surgeon applies mechanical force to the contact tip. For this reason, the protein coagulation layer and thermally damaged layer at the cutting surface become s thick.
Therefore, the erbium-doped YAG (Er:YAG) laser which has an oscillation wavelength of 3 xcexcm, at which the absorption coefficient with respect to organic soft tissue is highest has been developed, enabling precise cutting of hard tissue as well as soft tissue without applying any thermal damage to the areas around the exposed portions. However, since the excitation method for the Er:YAG lasers such as are currently available on the market is exclusively flash lamp excitation, the proportion of 3 xcexcm laser light capable of being generated by the level of photon energy outputted by flash lamps is rather small, so small in fact, that the electro-photonic conversion efficiency, i.e. the ratio of the laser output to the power consumed by the device overall, is extremely low at less than 1%. Additionally, since flash lamp excitation involves making the lamp emit sudden bursts of light by applying several kilovolts of electricity, electromagnetic noise is generated, which increases the danger that electronic devices used for the surgical operation or implanted inside the body of the patient will malfunction.
In search of a method of generating high intensity light at an oscillation wavelength of 3 xcexcm, research has gone into the development of semiconductor laser (LD) pumped Er:YAG lasers. While the operational modes include continuous wave and pulse types, in both cases, the problem of heat generated within the Er:YAG crystals has been an obstacle to the attainment of high outputs, so that the highest output power for a 3 xcexcm laser which has been confirmed has been 1.2 W.
Additionally, as other forms of 3 xcexcm light generating technology, a technique of wavelength-converting the output of an LD-pumped Nd:YAG laser with an oscillation wavelength of 1 xcexcm to 3 xcexcm light by means of non-linear optical crystals has been researched. According to W. R. Rosenberg et al., Optics Letters vol. 21, no. 17, p. 1336, 1996, an output of 3.55 W was achieved in continuous wave (CW) mode at a wavelength of 3.25 xcexcm. Furthermore, L. E. Myers et al., IEEE Journal of Quantum Electronics, vol. 33, no. 10, pg. 1663, 1997 describes that the fundamental harmonic beam in the output of an LD-pumped Nd:YAG laser which was Q-switched at a repetition frequency of at least 30 kHz was wavelength-converted to 3-4 xcexcm, thereby achieving an output of at least 3 W in this wavelength band.
The CW mode and Q-switched pulse mode are problematic for tissue removal treatments of hard tissue (especially dentin). In the CW mode, the energy density that is applied is inadequate to decompose enamel and dentin which are types of hard tissue, but the irradiated portions are thermally damaged. Furthermore, in the Q-switched mode, the pulse energy is small but the pulse width is short, thus resulting in a high peak power such that the heat due to the generation of plasma upon irradiation can damage even hard tissue. Additionally, with regard to the light generating devices, in the Q-switched mode, the peak of the fundamental beam pulse is so large that the end surfaces of the non-linear optical crystals used as wavelength converting portion can be damaged.
The present invention has the purpose of achieving higher operational efficiency, suppressing the generation of electromagnetic noise which can have a detrimental influence on nearby electronic devices, making the devices more compact, reducing damage to organic tissue in the areas around the irradiated portions, preventing damage to non-linear optical crystals and enabling multi-functional treatments with a single laser treatment device.
With the laser device according to the present invention, a laser pulse emitted from a solid-state laser pumped by a quasi-continuous wave LD (QCW-LD) is taken as the fundamental beam, and light of a desired wavelength is created from this fundamental beam by means of non-linear optics. The pulse width of the laser pulse in time is on the order of sub-microseconds to milliseconds, and transient oscillations are minimized by changing the waveform of the drive current sent to the QCW-LD. This waveform is a deformed rectangular wave which reaches a maximum value and a minimum value prior to plateauing. In order to achieve higher efficiency in the device by means of high efficiency wavelength conversion, a periodically poled magnesium-doped lithium niobate crystal is used as the non-linear optical element. Furthermore, an OPO resonator using two OPO mirrors and an optical element for complete internal reflection is used. Also, beams of a plurality of wavelengths wavelength-converted by the non-linear optical element and the fundamental beam are simultaneously or independently used.
With a quasi-CW mode LD (QCW-LD) pumped format, it is possible to achieve high efficiency, reduced electromagnetic noise and miniaturization. Additionally, the higher efficiency of the wavelength converting portion results in higher efficiency for the device overall. By making the pulses generated by the wavelength-converted LD longer, plasma generation at the irradiated object is suppressed and the damage to the areas surrounding the irradiated portions is minimized. At the same time, by controlling the waveform in time of the QCW-LD pulse, the transient oscillations can be suppressed while obtaining the same level of energy as in the conventional technologies, thus reducing the peak power in the device and prolonging the life of the device.
By irradiating simultaneously with a plurality of beams of different wavelengths obtained by wavelength conversion, it is possible to obtain a combination processing effect. For example, when the object of irradiation in a medical application is soft organic tissue, it is possible to achieve the simultaneous action of a sharp incision by the wavelength corresponding to the absorption peak of the tissue and hemostasis by wavelengths longer than the absorption wavelength having lower absorption coefficients. Furthermore, by using secondary visible light components generated by wavelength conversion to guide the beams of wavelengths not visible to the eye, it is possible to offer a device that is suitable for application to medical treatments.
The present invention offers a compact, high-efficiency light generating device with rediced electromagnetic noise suitable for medical applications. Since this single light generating device is capable of outputting a plurality of beams of different wavelengths, it is possible to achieve a combination of functions. Additionally, the present invention can of course also be applied as a light source for industrial machining applications, photo fluorescence diagnosis and photo-acoustic diagnosis.